go/types, types2: assert that builtin reports valid operand mode upon success

go/types, types2: assert that builtin reports valid operand mode upon success

Fix a case where x.mode == invalid was returned despite builtin
returning true.

Change-Id: Iae9c18aac16bcbadc3530d341b380e05c8743fcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/495299
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>